Children receive aerosol therapy on Wednesday at Beijing Children's Hospital, which has seen an increase in respiratory system diseases among youngsters since smog hit the capital this week. FENG YONGBIN/CHINA DAILY

At 5 pm on Wednesday, the reading for PM2.5 in Beijing was 233, still four times more than the safety level, the municipal environmental watchdog said. The smog is expected to persist until noon on Thursday when wind is forecast to disperse it.

Wang Zifa, a researcher at the Institute of Atmospheric Physics at the Chinese Academy of Sciences, said the emergency measures had cut pollution.

During the smog alert, 70 percent of pollutants had been generated from Beijing and surrounding areas, he said.

The capital has suspended or stopped production at about 2,100 polluting companies and outdoor work has been halted at 3,500 construction sites.

In inspections at 528 polluting companies and construction sites on Wednesday, the Beijing environmental watchdog found 37 violating the restrictions.

On Tuesday, the smog blanketed at least six provinces and municipalities, covering more than 840,000 square kilometers, the Ministry of Environmental Protection said.

In addition to Beijing, Tianjin and 29 cities in Hebei, Shandong and Shanxi provinces have issued different levels of alert aimed at curbing the smog.

This has reduced coal consumption and vehicle use in many cities.